The present invention relates to an organic electroluminescence element (hereinafter referred to as an organic EL element), and more particularly relates to the material of its organic compound layer.
The recent rapid development in information media has created a large demand for display device innovation to enable rapid and precise conveyance of as much information as possible. In this present-day situation, self-emissive organic EL elements, which possess features such as high-speed response, high luminance, low power consumption, and reduced occupying space, have gained attention for use as elements in next generation flat panel displays and planar light sources, and much research has been directed to organic EL elements.
An organic EL element using organic compounds in its emissive layer is characterized in that it can emit light of a high luminance. By applying a direct current of several volts to a thin film element composed of a metal cathode and a fluorescent organic layer having a thickness of only about 100 nm formed on a transparent anode, a large current close to 1 A/cm2 can be made to flow in the element. Efforts are being made to put such elements to practical use.
However, organic EL elements have not yet achieved sufficient stability and durability. Improvements in these points are indispensable for competing with of other types of displays.
It is known that the stability of the film structure of the organic thin film constituting the organic EL element significantly relates to the stability and durability of the element as a whole. In general, it is desirable that the organic thin film be made of a material that can be formed in an amorphous state and can maintain its amorphous state in a stable manner. However, crystallization begins to occur in an organic compound from the amorphous state when the glass transition temperature Tg is exceeded. Molecular movement is then activated, and the organic compound becomes unstable. Accordingly,to obtain a more stable organic thin film, it is necessary to develop a material having a high glass transition temperature Tg in addition to a high melting point Tm, and excellent heat resistance.
Four examples shown in FIG. 1 are representative configurations of the presently known organic EL elements. The optimal element configuration for the organic compound layer between a cathode and an anode differs depending on the characteristics of the employed organic material. For example, in the element of FIG. 1(a), a single emissive layer (EML) is disposed between a cathode and an anode, and this emissive layer also serves the functions of an electron transport layer (ETL) and a hole transport layer (HTL). In the element of FIG. 1(b), the emissive layer simultaneously serves as the electron transport layer. The hole transport layer supplies holes into the emissive layer to generate light emission. In the element of FIG. 1(c), the cathode supplies electrons to the electron transport layer. The anode supplies holes to the emissive layer which also functions as the hole transport layer. Light emission is generated near the interface between the emissive layer and the electron transport layer. In the element of FIG. 1(d), electrons are supplied to the emissive layer from the electron transport layer, and holes from the hole transport layer. The electrons and the holes recombine within the emissive layer to emit light. Presently, appropriate organic compounds are being proposed for the organic layers of elements having these various configurations.
The organic compound layer constituting the organic thin film is composed using, as referenced to above, a compound having hole transport function, a compound having electron transport function, and a compound having emissive function. Though it is desirable that one compound possess all of these characteristics (see FIG. 1(a)), usually a plurality of compounds are overlapped to form the organic compound layer (see FIG. 1(b)-(d)).
A representative hole transport material is an aromatic amine compound. Especially, dimer of triphenylamine, TPD (triphenylamine dimer), is known as an exemplary hole transport material. TPD can be easily formed on a substrate as a uniform amorphous thin film by vacuum deposition. However, there is a problem with TPD in that its glass transition temperature Tg is low, at 60xc2x0 C., and that TPD crystallizes even at room temperature after a long time, changing into an irregular film. Such a change in the film structure resulting from crystallization directly influences the life of EL elements. Provision of a hole transport material capable of maintaining a stable film configuration and having high glass transition temperature Tg is for this reason desired.
The same can be written about electron transport materials. Compounds including oxadiazole (PBD, BND) and triazole (TAZ) structures are known as electron transport materials. However, many of these materials also have low glass transition temperatures Tg and tend to crystallize. It is therefore difficult to achieve a stable element when these materials are used as the electron transport materials. Other problems of these materials, such as the requirement for a high drive voltage and insufficient durability, have also been pointed out.
Methods are recently proposed for raising the Tg of materials constituting an organic thin film. Such methods include introducing branches and non-planarity in the compound molecular structure to reduce intermolecular aggregation strength, thereby suppressing crystallizing property. Increasing the molecular weight is another of such methods. Polymers having starburst, Spiro, or linear structures, for example, are representative compounds obtained by those methods. A Spiro structure especially creates an extremely non-planar molecular structure, and use of this structure allows development of materials having high heat resistance. The Spiro compounds of tryphenylamine, oxadiazole, and oligophenylene, for example, can be hole transport, electron transport, and emissive materials, respectively.
Recently, a compound integrating spiro structure, in which branches and non-planarity are introduced into the molecular structure to reduce intermolecular aggregation strength and to thereby decrease crystallization property was presented by Hoechst (Polymer Preprints 38 (1997) 349). The compound presented here has the configuration denoted by the chemical formula below wherein two identical structural units are bonded, and demonstrates only one type of property. 
The above Spiro compound shows excellent structural stability, but lacks flexibility (allows for little variety) in its molecular structure. Moreover, this material is insufficient in terms of its electronic property. When the basic unit of the material has electron transport property, the material only demonstrates electron transport property. The material only functions as an emissive material when the basic unit is emissive. It is therefore necessary to separately develop new compounds exclusively for each of the properties.
The present invention was created in the above light. The object of the present invention is to provide a new organic material having high heat resistance, into which properties such as electron transport property, hole transport property, and emissive property can be freely integrated as the property of the organic compound.
The electroluminescence element of the present invention is an organic EL element having an anode, a cathode, and one or more organic compound layers sandwiched between the anode and cathode, wherein at least one layer of the organic compound layers includes an organic compound having the structure in which two biphenyl derivatives are bonded, as denoted by chemical formula (1). 
However, in the present invention, the above [A] does not include a structure composed of only a single carbon atom. In other words, the compound of chemical formula (1) does not include the structure in which two biphenyl derivatives are directly bonded in a Spiro bond. Specifically, the above [A] includes (i) two or more carbon atoms, (ii) a combination of one or more carbon atoms and a desired substituent or atom other than carbon, or (iii) a structure in which two biphenyl derivatives directly link at a plurality of sites without a bridging atom. In an organic compound having such a structure, adjacent substituents of the two biphenyl derivatives interfere with one another and generate steric hindrance. Accordingly, the two biphenyl derivatives do not locate themselves on one plane, and generate a twisted non-planar structure. Alternatively, while the two biphenyl derivatives are located within one plane, the substituents are positioned in a structure twisted from the plane. As a result, intermolecular aggregation strength can be reduced, providing a highly stable compound having reduced crystallizing property, a high glass transition temperature, and a high melting point.
Further, the organic EL element according to the present invention may include in at least one of the organic compound layers an organic compound given by the following chemical formula (2) 
which can be described as the above chemical formula (1) having a bond formed at [A] via ketone (carbonyl group).
The compound of the above chemical formula (2) is a spiro compound having asymmetrical basic skeletons. One of the basic skeletons constituting the asymmetrical Spiro compound is a fluorene skeleton, while the pairing skeleton comprises a molecular structure other than fluorene skeleton, thereby forming an a symmetrical spiro structure. According to this spiro structure having asymmetrical basic skeletons, the compound of chemical formula (2) can be easily used as a variety of organic materials for electroluminescence elements by replacing desired groups in chemical formula (2) (R1-R4, for example) with substituents appropriate for the usage of the material.
In addition, the chemical substance of chemical formula (2) has a molecular structure into which branches and non-planarity are introduced to reduce intermolecular aggregation strength and to thereby decrease crystallization property, and is a stable compound with high glass transition temperature.
Still further, the skeleton of the structure pairing with the fluorene skeleton in chemical formula (2) may include cyclohexanone. When such a compound is employed in the organic material layer of an organic electroluminescence element, the presence of carbonyl group in cyclohexanone enhances the adhesion property of the organic material layer with respect to the transparent electrode, made of ITO (indium tin oxide), for example, used as an electrode, allowing improvement of element durability.
According to another aspect of the present invention, at least one of the organic compound layers of the organic EL element includes an organic compound expressed by the following chemical formula (3). 
The compound of chemical formula (3) has a structure in which [A] of the above-mentioned chemical formula (1) is a double bond and the two biphenyl derivatives are bonded via this double bond.
The compound of chemical formula (3) is configured to include a widely extended and developed conjugated system. In such a compound having a developed conjugated system, xcfx80 electron system extends in the entire molecule. Accordingly, high emission efficiency can be accomplished, and excellent performance can be expected with regards to hole transport and electron transport functions.
Another feature of the present invention is that at least one of the organic compound layers of the organic EL element may include an organic compound expressed by the following chemical formula (4). 
The compound of chemical formula (4) has a structure in which [A] of the above-mentioned chemical formula (1) includes a third biphenyl (which may be its derivative), providing a three-dimensional structure. The compound therefore has an extremely high glass transition temperature Tg. By using this compound, it is possible to achieve a stable organic compound layer which is unlikely to crystallize even when formed into a thin film.
A further feature of the present invention is that an organic compound expressed by chemical formula (1), or more specifically, by any one of chemical formulas (2), (3), and (4), can be used as an emissive layer material having hole transport function.
According to the present invention, by introducing desired substituents in R1-R4, for example, an organic compound simultaneously having hole transport function and extremely high emission efficiency can be obtained. Such an organic compound having hole transport and emissive functions may be used to form, together with an electron transport layer, an organic compound layer of an organic EL element in a two-layer configuration. In such a two-layer configuration, due to the presence of the electron transport layer between the emissive layer and the cathode, excitons generated by the recombining of electrons and holes within the emissive layer are less likely move through the electron transport layer to reach the cathode. This in turn prevents excitons from being lost at the interface with the cathode without contributing to light emission. Accordingly, higher emission efficiency can be achieved in this configuration compared to a case when light emission is performed in an emissive layer that also serves as the electron transport layer.
In the above chemical formulas (1)-( 4), substituents having desired properties can be introduced as substituents R1-R16 (additionally as R17 and R18 in chemical formula (4)), and especially in R1-R4, to allow a single organic compound to possess a plurality of functions such as hole transport and emissive functions as described above, or electron transport and emissive functions.
A still further feature of the present invention is that the organic compound expressed by any one of chemical formulas (1)-(4) may be used as a host material within an emissive layer constituted by injecting a doping material within the host material.
The compound can be used as the host material of an emissive layer, as mentioned above, through selection of substituents R1-R4 to be introduced. Appropriate selection of materials allows adjustment of emitted color and improvement in emission efficiency, resulting in high efficiency organic EL elements capable of emitting light with a high luminance.
According to another aspect of the present invention, an organic EL element comprises a first emissive layer including, as an emissive material, an organic compound expressed by any of the above chemical formulas (1) to (4), and a second emissive layer including a material different from the material of the first emissive layer, wherein combined light including light from the first emissive layer and light from the second emissive layer is used as the light emitted by the element.
By allowing light from the first and the second emissive layers to coexist as mentioned above, a desired mixed color can be achieved. For example, white light emission may be generated employing the organic compound of the present invention and another compound.
A further feature of the present invention is that at least one of the organic compound layers of the organic EL element may include an organic compound expressed by the following chemical formula (5). 
The compound of chemical formula (5) comprises an orthotetraphenylene skeleton, which can be described as the above chemical formula (1) having no atom in [A], in which two biphenyl derivatives are directly bonded to one another in a plurality of sites (two sites).
A still further feature of the present invention is that the organic compound expressed by chemical formula (5) may be used as a hole transport layer material for transporting holes from the anode to the emissive layer.
In the compound of the present invention expressed by chemical formula (5), groups having desired properties can be introduced especially as substituents R1, R2, R3, and R4 among the plurality of substituents R1-R16, to allow the compound to possess hole transport property, emissive property, or electron transport property. Moreover, a plurality of properties, such as the emissive property and hole transport property, can be imparted to the compound. Accordingly, this compound can easily be used as organic material for a variety of organic EL elements.
While example combinations shown in Table 1 described later can be applied to the substituents R1, R2, R3, and R4 in the compounds of the present invention expressed by the above chemical formulas (1)-(5), each of the substituents may be selected from hydrogen, alkyl group, alkoxy group, phenyl group, substituted phenyl group, diphenylamino group, diarylamino group, and other groups such as heterocyclic group and substituted heterocyclic group. No particular restrictions exist for other substituents R5-R16 in chemical formulas (1)-(5) (additionally for R17, and R18 in chemical formula (4)).
In the organic compounds expressed by chemical formulas (1)-(3) and (5), the two biphenyl derivatives generate steric hindrance due to the presence of the substituents R9 and R11, and R10 and R12 (in the organic compound of chemical formula (2), the steric hindrance is mostly due to the closely located R9 and R11). The two biphenyl derivatives are therefore prevented from being located within one plane. In addition, R14 and R15, and R6 and R7 also generate steric hindrance. In this way, the organic compounds expressed by chemical formulas (1)-(3) and (5) possess a twisted non-planar structure. This non-planar structure reduces intermolecular aggregation strength, thereby decreasing crystallizing property. Further, high glass transition temperature Tg and high melting point Tm are achieved, providing stable compounds having excellent heat resistance. Accordingly, stability of the organic layer can be enhanced by using these organic compounds, either alone or in combination with other organic compound materials, as the material for the organic layer. Furthermore, the organic compound of chemical formula (4) having a three-dimensional structure as described above is non-planar and has high heat stability without depending on R1-R16, R17, and R18.